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• Wrong colors when converting an AVFrame to QVideoFrame

  2 janvier 2020, par Michael G.

  I read videos with libav and display them in a QAbstractVideoSurface in QML. It works so far, however, I did not manage to get the colors right.

  My av_read_frame Loop looks like this :

  if (frameFinished)
  
  {
  
                          SwsContext* context = nullptr;
  
                          context = sws_getContext(_frame->width, _frame->height, (AVPixelFormat)_frame->format, _frame->width, _frame->height, AVPixelFormat::AV_PIX_FMT_RGBA, SWS_BICUBIC, nullptr, nullptr, nullptr);
  
                          QImage img(_frame->width, _frame->height, QImage::Format_RGBA8888);
  
                          uint8_t* dstSlice[] = { img.bits() };
  
                          int dstStride = img.width() * 4;
  
                          sws_scale(context, _frame->data, _frame->linesize,
  
                              0, _frame->height, dstSlice, &dstStride);
  
  
  
                          av_packet_unref(&packet);
  
                          sws_freeContext(context);
  
  }

  If I save the image to disk at this point, the colors are already wrong (everything looks red).
  Later, I display the images in a video surface with the format QVideoFrame::Format_ARGB32, and the colors are wrong again, but look different than the saved image (everything looks blue).

  I started to experiment with libav/ffmpeg recently, so maybe the problem is something else and I just have no clue. Let me know, if you need more information :)

• Jerky Video from Jpgs encoded with ffmpeg [closed]

  16 février 2012, par Glstunna

  I have a bunch of jpg snapshots taken from a 3d program at perfect time frames.
  But when I have the videos compiled/encoded into a video by ffmpeg, I notice an annoying jerkiness. The jerkiness is not that pronounced, but enough to be annoying, especially during slow camera pans.

  This is what I use :

  "ffmpeg-lgpl.exe" -y  -r 29.97 -i "C:\vidsnaps\vid_%d.jpg" -b 8000k "C:\Users\peki.ICE\Documents\macbattle.mpg" 

  I chose mpg (mpeg1video) because that is the format readily available in all end-user systems like XP without downloading extra codecs. The video images are guaranteed to match that framerate 29.97 as the camera in the 3d program pretty much waits for each frame to be dumped to file before moving to the next one.

  What other fancy ffmpeg flags do I have to set for this thing to stop being jerky.

  EDIT : see video example here and notice the light jerk/stuttering.
  http://www.youtube.com/watch?v=g-slsJBZA2w&feature=youtu.be

• FFMPEG : How to overlay a large size png on jpg [closed]

  21 juin 2022, par Yoav Mor

  I'm trying to use ffmpeg to achieve the following :
x.jpg = a file that is 540x540 in size.
Gradient.png = a 540x960 image that has a gradient of semi transparency (which starts with dark blue with no transparency in the top and then gradually becomes transparent in the bottom)
example of Gradient.png attached

  


  The goal is to end up with a jpg that is 540x960, where x.jpg is in the bottom 540x540 of the frame and the Gradient.png is laid on top of it. Because Gradient.png is larger, this seems to pose a problem.

  


  I tried :

  


  ffmpeg.exe -i x.jpg -i Gradient.png -filter_complex overlay=0:480 -c:v png -pix_fmt rgba out.png


  


  Which just creates a 540x540 frame, so doesn't work. Feels close though. If I new how to enlarge the "canvas" this could have maybe worked.

  


  I tried.

  


  ffmpeg.exe -i Gradient.png -i x.jpg -filter_complex overlay=0:480 -c:v png -pix_fmt rgba out.png


  


  Which does create a 540x960 frame but Gradient.png is not on top of 1.jpg, the opposite is happening here, so I don't get the effect of the semi transparency shown on top of x.jpg like I need.

  


  Any help will be highly appreciated

  


  Edit :
I was able to do it in 2 steps.
ffmpeg -i x.jpg -vf "scale=540:960:force_original_aspect_ratio=decrease,pad=540:960 :(ow-iw)/2 :(oh-ih)" -c:v png -pix_fmt rgba out.jpg
ffmpeg -i out.jpg -i Gradient.png -filter_complex overlay=0:0 OUT2.jpg

  


  If someone can come up with a way to do it in one step, it'll be great.